The present invention relates to a pumping device and, more particularly, to a recirculating pump device for anti-locking motor vehicle brake systems, with an anti-locking device operating in accordance with recirculating principles.
A recirculating pumping device of the aforementioned type is proposed in, for example, German Patent Application P No. 29 44 856.4, wherein the device provides for cyclic monitoring of the state of movement of the braked wheels and, when a wheel develops a tendency to lock up, causes the brake fluid to be released from an affected wheel brake cylinder, by switching an anti-locking safety valve, as well as admitting the brake fluid, released to the lower pressure, into the brake line to be recycled, whereby one piston pump is associated with the brake circuits of the vehicle within the recirculating pumping device. A feed pressure chamber of the piston pump is connected through an inlet check valve to a recirculating line segment communicating with the wheel brake cylinders of the corresponding brake circuit, and through an outlet check valve to a brake line of the corresponding brake circuit. The drive of the piston pump is controlled by a switching valve, by means of which the working pressure chambers of the piston pumps are connectable at a predetermined switching frequency to a pressure source provided on the vehicle, or, alternatively, may be vented into a tank or reservoir of the pressure source.
In a special type construction, the recirculating pumping devices generally include recirculating pumps constructed as hydraulically driven piston pumps, with one of the pumps being associated with the front axle brake circuit and another pump being associated with the rear axle brake circuit. The effective piston surfaces of the piston which delimits the feed pressure chamber from the working and/or drive pressure chamber are of an equal size. The feed pressure chambers of the piston pumps are each connected by an inlet check valve with a segment of the brake line branch leading to the corresponding wheel brake cylinder. The segment of the brake line branch is cut off from the main brake cylinder in a regulating phase of the anti-locking device of the vehicle brake system by a corresponding brake-pressure regulating valve and connected to a segment of the brake line which leads back to the main brake cylinder by an outlet check valve. The working pressure chambers of the piston pumps are connected alternately to the tank and/or a pressure outlet of a hydraulic auxiliary pressure source in a regulating phase of the anti-locking device by an appropriate control of a switching valve which may, for example, be constructed as a solenoid valve. The corresponding recirculating pump executes a filling stroke while its working pressure chamber is connected to the tank of the auxiliary pressure source through the switching valve, under the influence of a pretensioned piston spring. The spring has a spring force which is exerted on the piston to enlarge the feed pressure chambers. The piston executes a recirculating or working stroke against the restoring force of the spring and is triggered by the connection of the working pressure chamber to the pressure outlet of the auxiliary pressure source, under the control of the switching valve, whereby the anti-locking device may execute a plurality of filling and feed strokes of the recirculating pump within a single regulating phase if such is required.
Since the first feed stroke of the piston of the corresponding or respective recirculating pump which occurs within a regulating phase of the anti-locking device operates against a pressure which corresponds approximately to the instantaneous braking pressure in the wheel brake cylinder in the above-noted proposed recirculating pumping device, and since the first feed stroke additionally is made against the restoring force of the piston spring, the auxiliary pressure source in the proposed construction must be designed for at least a starting pressure of about 200 bars which corresponds to the maximum braking pressure.
Since hydraulic auxiliary pressure sources which may be available on a motor vehicle and provided such as, for example, power steering or load-level regulation, are generally constructed for a much lower starting pressure of, for example, 60 to a maximum of 150 bars, the existing hydraulic auxiliary pressure sources cannot readily be utilized to drive the recirculating pumps and, consequently, it is necessary for an additional auxiliary pressure source to be provided to drive the recirculating pumping device, with such auxiliary pressure source delivering the necessary initial pressure of approximately 200 bars.
In order to be able to attempt to utilize an auxiliary pressure source which is already normally provided on a motor vehicle and which has an initial pressure lower than the maximum braking pressure to drive the piston pumps, at least one additional pressure transducer and high pressure reservoir chargeable thereby in addition to a storage load control arrangement are required. As can readily be appreciated, the associated technical expenses for providing the additional components is considerable. Consequently, a recirculating pumping device of the above-proposed construction does not produce any significant savings over a conventional recirculating pump assembly driven by an electric motor.
A further disadvantage of the above-proposed recirculating pumping device resides in the fact that each working stroke of the one or more recirculating pumps always recirculates the same volume of brake fluid into the brake line, that is, the volume corresponding to the intake volume of the pump. Consequently, at the end of a regulating cycle of the anti-locking device, as a rule, more brake fluid must be drawn out of the pressure-regulating wheel brake cylinder than would be required for an appropriate anti-locking regulating effect.